In the semiconductor industry, the recent finer designs of semiconductor devices have led to a need for finer patterning than the transfer limit according to a photolithographic process. Extreme ultra violet (hereinafter, EUV) lithography is a promising exposure technique which uses EUV light having shorter wavelengths, in order to realize transferring of such fine patterns. Here, the term “EUV light” indicates light having wavelengths in the soft X-ray region or the vacuum UV region, specifically, wavelengths of about 0.2 to 100 nm.
Reflective masks used in this EUV lithography generally have a structure in which a multilayer reflective film to reflect EUV light and an absorber film to absorb EUV light are sequentially disposed on a substrate such as glass or silicon, and a protective film is disposed between the absorber film and the multilayer reflective film in order to protect the multilayer reflective film during the formation of a transfer pattern in the absorber film.
As mentioned above, a prescribed transfer pattern is formed in the absorber film. In an exposure machine carrying out pattern transferring, the light incident on the reflective mask is partly absorbed by the absorber film pattern and is partly reflected by the multilayer reflective film exposed from the absorber film pattern. The reflected light image is transferred onto a workpiece such as a silicon wafer through a reflective optical system.
In an exemplary method for the formation of a transfer pattern in the absorber film, a resist pattern is formed on the absorber film of the EUV reflective mask blank in which the multilayer reflective film, the protective film and the absorber film are formed in this order on the substrate; the absorber film is then etched by dry etching or the like while using the resist pattern as a mask; and the resist pattern is removed. (When, for example, the absorber film is a Ta-containing material, the absorber film pattern is formed by dry etching with a Cl-based gas.)
In the above method, over-etching is carried out to some degree to ensure that the absorber film will be completely patterned. As a result, the film disposed under the absorber film will be etched as well. Thus, a protective film is generally provided on the multilayer reflective film to prevent the surface of the multilayer reflective film from being damaged by etching. Protective films including ruthenium have been proposed for this protective purpose (Patent Literature 1). Further, protective films including a Ru alloy of ruthenium with zirconium and boron have been proposed (Patent Literature 2) in order to suppress the formation of a diffusion layer between a Si layer defining the surface of a multilayer reflective film and the protective film (the formation of such a diffusion layer leads to a decrease in the reflectance of the multilayer reflective film).